Our goal is to relate morphological and in vivo functional properties of elderly muscle to the performance in activities of daily living before and after strength or endurance training. We employ a new technique for examining in vivo muscle energetics, the Quantitative Energetics Stress Test (QUEST), and a new method for direct observation of performance in daily activities, the Physical Functional Performance (PAP) profile. With nuclear magnetic resonance (31P NMR) spectroscopy of limb muscle (vastus lateralis) under maximal stimulation conditions, we will use creatine phosphate (PCr) changes to measure both ATp breakdown and recovery. Muscle energetics will be evaluated as they relate to the myosin isoform composition (assessing PCr breakdown) and capillary and mitochondria densities (assessing PCr recovery) from muscle biopsies of the vastus lateralis. These new techniques will be combined with established methods. Muscle strength will be characterized by isokinetic tests of maximal force production (peak torque), and with NMR imaging to determine muscle cross-sectional area. Physical Functional Performance will be evaluated by direct observation of performance in daily living tasks (PAP), of gait and balance, and of reaction time. Muscle properties will be correlated with functional performance measures. In pre-trained elderly subjects, we first establish the relationships between in vivo muscle function the morphological and biochemical properties of elderly muscle. Second, we examine how performance (e.g., stair-climbing) depends on these in vivo muscle functional properties (e.g., muscle strength). On the basis of these baseline relationships, we will evaluate how performance and the underlying in vivo properties of the muscle in elderly subjects are uniquely altered by specific exercise training.